conjugated polymer nano-composites

ELSEVIER

SyntheticMetals 76 (1996) 289-292

Novel inorganic/conjugated

polymer nano-composites

Patricia G. Hill, Peter J.S. Foot, Reg Davis School of Applied

Chemistry,

Kingston

University,

Penrhyn

Road, Kingston-upon-Thames,

Surrey

KTl 2EE, UK

Abstract Inorganic/conjugated polymer nano-composites have been synthesized by a two-stage process involving intercalation of aromatic bases into an inorganic host. Oxidation led to in situ polymerization to produce alternating inorganic/polymer layers, and the transport properties of these complexes are compared with those of the original materials. ‘Soft chemistry’ techniques have been used, with some success so far, to isolate the free polymers for separate study. Keywords:

Nano-composites;Polymerization;Intercalation

1. Introduction The need to find conducting polymers with high chargecarrier mobilities has generated considerable interest over the last decade. However, prototype polymer devices, such as MISFETs [l] have revealed mobilities at least four orders of magnitude lower than that in crystalline silicon. In films of evaporated conjugated oligomers [ 21 mobilities comparable with that of amorphous silicon have been obtained, some two orders of magnitude higher than those for conducting polymers. There is obviously a need to improve transport properties further. For this, single-crystal conducting polymers would be ideal, but are not generally producible. A promising strategy is that of intercalating polymers into a two-dimensional host, so that they are held in an ordered matrix, to favour the development of high crystallinity. The process of intercalation into inorganic hosts has been well documented [ 3-61; it may either be accompanied by electron transfer [7] in hosts such as TaS, or by cation exchange in materials such as CdP& [ 81. Attempts to intercalate polymers from solution into inorganic hosts have been made, for example, polyethylene glycol or oxide [ 91, which have also been intercalated in V205 xerogels [lo]. Polyaniline and other conducting polymers have successfully been formed within an FeOCl host structure [ 1 l-141, but they generally yield poorly crystalline products. This host is an oxidizer and is non-stoichiometric (with interlayer Fe*+), and hence the monomer is partially polymerized before it can become well ordered between the layers [ 151. The hosts in this work have been chosen for their nonoxidizing properties, for their availability or ease of prepa0379-6779/96/$15.000 1996Elsevier ScienceS.A. All rights reserved

ration [ 16,171, their resistance to both reduction and oxidation and their versatility as host materials [ 18-211. Of the ternary compounds, Nip& is a wide gap semiconductor (,?$ N 1.6 eV) , while CdP& is an insulator (E, N 3 eV) . Both have monoclinic structures with strong intralayer and weak interlayer bonding [ 161. Moo3 is an insulator and has a hexagonal structure in which distorted Moo6 octahedra share edges and vertices to give strongly bonded Moo3 sheets, separated by a van der Waals gap. Either molecules with high basic strength or low ionization potential are good candidates for intercalation [ 181. Our aim in the present research was to produce crystalline conducting polymer complexes via self-organization of intercalated monomers, and to investigate their optical and electrical properties. ‘Soft chemistry’ has been used to extract and study the resultant polymers in some cases.

2. Experimental Polycrystalline CdPS, and NiP& were prepared by direct combination of the ultra-pure elements as described by Klingen et al. [ 161, and Analar grade Moo3 (Merck) was used as supplied. Particle sizes were less than 45 pm. Intercalation of aromatic/heterocyclic monomers into the inorganic hosts was achieved in several ways, for example: 1. Direct intercalation of a solid organic species into the host: A stoichiometric excess of 2,2’-bipyridyl ( 1 g) and NiP& ( 1 g) was sealed in an evacuated ampoule and heated for 4 weeks at 110 “C. The resultant solid was washed with ethanol and ether, and dried under vacuum.

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2. Direct intercalation of a liquid organic species into the host: An excess of pyridine (4 g; 60 “C), pyridazine or pyrrole (4.5 g; ambient temperature) was added to CdPS? ( 1 g), purged with N2 and stirred for 4-6 weeks. Pyrrole (3.5 g) or pyridine (3 g) was likewise reacted with NiP& (1 g). Products were filtered, washed with ethanol and ether and dried. 3. Intercalation of the organic species by two-stage methods: (a) Terthiophene (0.5 g) was exchanged for pyridinepreintercalated into CdP& (0.48 g) . The two solids were sealed in an evacuated ampoule and heated at 110 “C for 4 weeks. All the pyridine was removed to give a pale gold terthiophene intercalate. (b) MOO, was intercalated with Na+ ions by reaction with an excess of saturated aqueous Na2S204. The hydrated sodium ions were then exchanged for homosolvated anilinium ions by treatment with an aqueous solution, containing both aniline and anilinium ions. The two-stage intercalation process for Na+ [ 151 ions and then aniline/anilinium ions, respectively, into MOO, can be summarized as follows: xNa++yH,O+xe-+MoO,+Na,+(H,O),[MoO,]”and zC6H,NH, + xC6H5NH3+ + Na,+ (H,O), [ MOO,] x- + (C6H5NH,f),(C,H,NH,),[Mo0,]“-

+xNa+ +yH*O

Powder X-ray diffractometry (XRD) coupled with simple molecular modelling (DTMM Version 2.0, Oxford University Press) was carried out to indicate the likely orientations of the molecules within the host lattice, and the feasibility of an ordered polymerization in situ. Where appropriate, the monomer intercalates were exposed to (mildly oxidizing) I2 vapour at ambient temperature to initiate polymerization. The products were characterized by Fourier transform infrared spectroscopy (FT-IR) to confirm the presence of the organic species, and the monomer complexes by thermogravimetry (TGA) to establish the ratio of volatile intercalant to host. Measurements of powder pellet d.c. conductivity (van der Pauw configuration) were made, before and after intercalation, after polymerization and after dissolution of the host where possible. A few polymers were extracted for separate study by using ‘soft chemistry’: MOO, lattices were decomposed by treatment with 4 M NaOH. The polymer was reprotonated by stirring in an acidified solution [ 221 before filtering off, washing and drying. CdP& intercalates were similarly reacted with EDTA disodium salt.

Results and discussion

In all cases where intercalation had taken place, XRD showed an expansion in the interlayer spacing. For NiPSa

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and CdPSa this was in the c direction, and for MOO, the b direction. The strong family of 001 lines for the ternary host compounds typically showed a much larger d spacing, and a slight shrinkage on polymerization. In a few casesthe original hkl lines were present in reduced intensity, suggesting an incomplete intercalation reaction. With the exception of terthiophene the expansion was consistent with the plane of the included organic molecules being perpendicular to the layers of the host material, and not parallel as expected [23], The intercalated MoOa showed a much larger expansion of the 020 spacing than expected, consistent with a bilayer of intercalated aniline/anilinium ions. This is by no means an unknown phenomenon [ 241. Data for the observed expansions of inorganic hosts are shown in Table 1, Further measurements were only made in cases where XRD showed that intercalation had been successful. Table 1 Interlayer expansions of intercalates from XRD data Host

Monomer

Expansion (A)

NiPS, NiPS, NiPS, CdPS, CdP& CdPSs CdPS, MOO,

pyrrole pyridine 2,2’-bipyridyl pyrrole pyridine pyridazine terthiophene aniline

none none 9.83 none 6.36 6.12 3.39 13.56

Expansion after oxidation (A)

9.58

6.04 3.20 13.26

NiPS,

(4

NiPS, CdPS, (b) CdPS, MoOz

j-J++

(c) MoOB

Fig. 1. Schematic representation of the orientation of molecules within a host lattice: (a) bipyridyl in NiPSa; (b) pyridazine in CdP&: (c) aniline and anilinium ions in MOO,. Key: grey is carbon; white is nitrogen; black is hydrogen.

P.G. Hill et al. /Synthetic

Table 2 Thermogravimetric data of intercalates Compound

Weight loss (%)

Mole ratio of inter&ant/ host

NiPS,/2.2’-bipyridyl CdPSaipyridine CdPSJpyridazine MoOJaniline

30 7 17 19

0.51 0.23 0.61 0.40

On the basis of molecularmodelling, the likely orientations of the molecules between the host layers are shown in Fig. 1 (a)-(c). For monomer intercalates, weight loss due to organic volatile content was measured by TGA and used to estimate the ratio of organic species to host. Table 2 shows the TGA data for the intercalates, Evidence of successful polymerization was given by an absence of weight loss below the decomposition temperature of the host. FT-IR spectra of the intercalated materials showed peaks consistent with the organic species as well as the host matrix. For the MP& intercalates, the strong P,S6 deformation at 580

I

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291

cm-’ showed two- or three-fold splitting, as previously seen for organometallic MPSa intercalates [ 191 and due to the local host distortion in the vicinity of the intercalant. Fig. 2 shows a typical spectrum; the presence of N-H stretching vibrations indicates that the monomer was partially incorporated in cationic form. This must arise from partial disproportionation reactions causing protonation of the bases rather than direct electron transfer to the host [ 181. Evidence for polymerization after oxidation was shown by a weakening of the C-H stretching intensity and the appearance of new C=N and C=C peaks. Both NiPSs and MOO, intercalate with electron transfer, while CdPS, always intercalates by cation exchange. Both Nip& and Moos are weaker electron acceptors than, for example, FeOCl or V,05, into which pyrrole will readily intercalate. Pyrrole has a rather low ionization potential compared with the other organic species used in this work, but is a much weaker base; hence it did not intercalate into the selected host compounds. Of the intercalated organic species, only pyridine failed to polymerize by oxidation. The modest conductivities of the,NiPS, and CdPS, intercalates (Table 3) are due to the insulating properties of the

vCH

VP-P

1000 cm-’ 500 3000 2600 2000 1500 Fig. 2. FT-IR spectrum of NiPS, intercalated with oxidized 2,2’-bipyridyl.

3500

Table 3 Conductivities of intercalates (pressed powder pellets) Host

Intercalant

Conductivity before oxidation (S cm-‘)

Conductivity after oxidation (S cm-‘)

Conductivity of isolated polymer (S cm-‘)

MOO, NIPS, CdPS3 CdPS3 CdPS,

aniline 2,2’-bipyridyl pyridine pyridazine terthiophene

1.9x 1o-4 4.5x10-13 8.0X 1o-g 9.4x lo+ -lo-”

4.74x 10-l 1.2x lo-‘* 8.5 x lo-lo 2.5 x 1o-6 8.42X 1O-3

2.87X10-”

1.8 x 10’

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P.G. Hill et al. /Synthetic

hosts, although the strong increase shown by terthiophenel CdPS3 after oxidation is evidence of polymerization. Conductivities generally were lower than for either single crystals or continuous polymer films, partly due to grain-boundary resistance and (in the intercalates) to random orientation of the anisotropic crystallites within the pellet. For MPS3 complexes before oxidation, electrode polarization effects showed that the conductivity was largely ionic; it became fully electronic after polymerization. Only the extractions of polyaniline and polythiophene have so far been successful. For the former, viscosity and spectral data suggested a fairly low molecular weight, pernigranilinelike product, and the conductivity was only moderate. The terthiophene polymer was intractable, but identifiable by IR spectroscopy; its conductivity of 18 S cm-’ after I, vapour doping was quite high for a pressed pellet sample of polythiophene. No attempt has yet been made to optimize the polymerization processes with respect to chemical structure or molecular mass, and work on this aspect is planned for the near future.

4. Conclusions A variety of aromatic/heterocyclic monomers has been intercalated into lamellar host lattices. Simple modelling has indicated the orientation of the molecules, and where polymerization was feasible this has been achieved by exposure to an oxidizing atmosphere. In all cases except pyridine, polymerization has taken place, accompanied by slight shrinkage of the interlayer spacing (XRD) , weakening of C-H stretch modes and the appearance of new C=N or C=C! peaks (FTIR spectroscopy). Conductivities were consistently higher for the polymerized intercalates. It has been possible to extract two conductive polymers from the host lattices, although the molecular mass, state of oxidation and level of protonation have not yet been optimized.

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